Garmin Circa gets Sleep Alignment but not Index Sleep Monitor?

I'm a bit confused by the way sleep alignment is implemented with the ISM. From what I understand, the ISM can collect the necessary data, but sleep alignment is only available when paired with a compatible watch, even if that watch isn't actually being worn during sleep.

Could Garmin explain the reasoning behind this requirement? I currently use an Epix Gen 2, which is still a very capable device, and recently added the ISM to my Garmin setup. It's disappointing to find that some sleep metrics are unavailable unless I upgrade to a newer watch.

If sleep alignment data is already being uploaded to Garmin Connect, it seems like this could potentially be presented there rather than requiring a specific watch model. Since the ISM itself doesn't have a display, Garmin Connect feels like a natural place for that information.

It also seems that users who purchase the ISM as their first Garmin device may not get access to the full sleep functionality, which could be confusing and might not provide the best introduction to the Garmin ecosystem.

I'd be interested in understanding whether there's a technical limitation behind this decision, or if there are plans to make these metrics available more broadly in the future.

One possibility that occurred to me is that Sleep Alignment may depend on inputs that aren't available on older hardware, particularly skin temperature data, and information capture is required all day, not just while sleeping.

If Garmin is using skin temperature, along with other physiological signals to infer circadian rhythm and determine an optimal sleep window, that would help explain why my Epix Gen 2 (with the Elevate Gen 4 sensor) doesn't support the feature.

If that's the case, can Garmin clarify whether skin temperature is a required input for Sleep Alignment? Understanding the dependency would make the current device support matrix much easier to understand.

It also raises an interesting question regarding devices with the Elevate Gen 5 sensor. If skin temperature is a key requirement, then it would seem that devices such as the Epix Pro and Fenix 7 Pro already have the necessary hardware and could potentially support Sleep Alignment if Garmin chose to enable it in software.

My other thought is that Sleep Alignment may require more than just overnight sleep data. If Garmin is attempting to model circadian rhythm rather than simply analyze sleep sessions, it would make sense that skin temperature and other physiological metrics need to be captured throughout the day as well. In that scenario, the ISM could provide the overnight sleep data, but a compatible watch would still be needed to collect the daytime inputs required for the calculation.  That would explain why a compatible watch is required, including the Cirqa with the Gen 4.1 Elevate sensor (Gen 4 plus skin temperature), but the ISM alone is insufficient.

If that's the technical reason behind the feature requirements, I'd be very interested to hear Garmin confirm it. Right now it's difficult to tell whether the limitation is due to sensor capabilities, firmware support, backend processing requirements, or a combination of all three.